Rainbow Dash is a Fortifier
Applejack trotted behind Rarity as she followed the script, waiting for Rainbow Dash to come zipping over to the five ‘normal’ ponies left in town. She had already seen the pegasus’s telltale contrail zooming all around, so it was only a matter of time before Dash made it to them. Although Applejack had been a little worried when Rainbow suddenly zipped up and went in the wrong direction a minute ago as if she was leaving Ponyville altogether, she had returned in short order.
Applejack didn’t know what she was so worried about. Despite her insensitivity, Rainbow wouldn’t just abandon Ponyville to some zombie plague.
If she did, well...it would have meant all that work Twilight and the others did to set up what was a truly masterful prank if Applejack had any say so had really gone to waste.
Then Rarity’s horn lit up, and she lifted the box of gag treats. “Anypony want a cookie?”
“DON’T TOUCH THOSE!”
The sudden appearance of Rainbow Dash that Applejack missed thanks to a blink had the farm pony needing to take a second to reevaluate the situation. Rainbow Dash sure did look panicked and all, but… “Uh, Rainbow?” AJ spoke up as she pointed to the very long and sharp-looking blade in the pegasus’s left hoof. The red staining along said blade sent a shiver down the farm pony’s spine. “W-What’re you doin’ with that there machete?” And is that...blood?
She couldn’t bring herself to ask the second part.
Rarity let out a gasp before she reached down to cover Sweetie Belle’s eyes while Rainbow blinked and slowly turned around. “Hmm? Oh this?” she asked while bringing up her weapon, allowing more of the red ichor to slowly run down it while her own expression fell. “Naw, it’s a super deluxe cake cutter. I had to uh...use it. On the Cakes.”
“Oh sweet Celestia no,” Applejack breathed in horror.
Rarity looked about ready to faint for real.
“Look, everypony who ate those stupid cookies is turning into some kind of freaky zombie!” Dash said before pointing to the crowd of Ponyville citizens with her free hoof.
Applejack gulped and turned around to look at the shambling mass of ponies that were coming out of their houses as part of the gag. The Cakes were indeed absent, as was… “R-Rainbow...where’s Pinkie Pie?”
“Hey!” Rainbow said in an indignant tone before getting in Applejack’s face. “I’m not going to let my best friend spend the rest of eternity as one of those trotting dead ponies! Come on Rarity. You’ve got tons of sharp scissors and stuff like that in your shop, right? Just go for the head, that takes them down easy. Now let’s get some weapons!”
Without waiting for the others, Rainbow opened the door to the unicorn’s shop and dashed inside.
“Applejack...she...she ki-”
“Uh, what’s goin’ on?” Scootaloo asked as she looked around. “I thought we were supposed to be running from the zombies.”
Applejack’s eyes widened in horror as she looked at the three innocent fillies. “Um...that’s...uh…”
Before the orange pony could think of some way to explain to the orange pegasus how her hero was not a multiple murder with three...no, Applejack suddenly realized. We were gonna put the cookie juice on the foals too.
Foalicide, Rainbow Dash had committed foalicide.
Rarity’s whole body shuddered. “Applejack...I think I’m going to be sick.”
“YOU DIDN’T EAT A COOKIE, DID YOU?” Rainbow Dash shouted as she suddenly zoomed back out the door with her machete poised to strike at Rarity’s skull.
The unicorn in question shrieked and jumped away. “No! No! NO!” Rarity shouted as she held up her hooves in defense.
Rainbow blinked before lowering her murder weapon. “Oh...well, I guess all those diets you go on finally payed off Rares,” she said before flying around to push the unicorn inside the boutique. “Now come on, we gotta fortify!”
Two minutes later, Applejack found herself looking out the window of Rarity’s Boutique as the mass of encroaching ponies looked more confused than hungry while they came ever slowly onward towards the unicorn’s house. Although the even more worrying sight was Rainbow Dash. She flew around with a bandoleer of makeshift daggers created from the blades of scissors that Rarity had around her shop.
“Okay so we’ve got throwing knives, fire, sheets of cheap fabric and the cooking oil to soak them in so we can burn the zombies to ash,” Rainbow said as she slowly flew back and forth across the room. “The real problem’s gonna be Twilight, if she can still use her magic, I mean. Can zombies use magic?”
Applejack gulped and looked back out the window towards said pony...who looked a little more ornery than the rest of the extremely slow moving horde of zombies as her plan continued to stall in between steps 235 and 236.
Rarity on the hoof, she was curled up in a little ball in the corner to Applejack’s left. “This isn’t happening. This isn’t happening. This isn’t happening. This isn’t happening.”
“OF COURSE IT’S HAPPENING!” Rainbow yelled at her as she zoomed down to glare at Rarity. Then, when the unicorn looked back up at her with wide eyes, Rainbow Dash became extremely hesitant before she dropped to the ground. “Uh...look...Rarity, I...um...Applejack, you talk to her! I’m gonna go check on the girls.”
As Dash disappeared up the stairs in a rainbow blur, Applejack trotted over to Rarity. “Why did I let you all talk me into this?” she muttered. “It wasn’t a bad joke. She gave me a cake. Two cakes! A MOUNTAIN OF A CAKE! WHY WOULD ANYPONY WANT TO TAKE REVENGE ON ANOTHER PONY WHO GIVES THEM A MOUNTAIN OF CAKE?”
Applejack gulped. Unable to think of anything to say, the orange mare’s mind latched onto the questions being asked of her. “W-Well, she was out of control Rarity, Ah mean...you saw how she was acting. She frightened Fluttershy.”
“EVERYTHING FRIGHTENS FLUTTERSHY!” the hysterical pony shrieked before she got in Applejack’s face with a wide-eyed expression. “Oh, but instead of going, hey maybe the pony that has to face down horrible monsters as an integral and irreplaceable part of Equestria’s first and best line of defense should learn to deal with being frightened from time to time, youuuuuuuuu go, ‘dur frighten’en’en’en’en’en’en’en’en’en’en’en’ Fluttershy, that’s just plum lazy Rainbow Dash, Uh-huck!”
“H-Hey! Ah don’t sound nothin’ like that!” Applejack replied before realizing she was getting sidetracked and quickly shook her head to clear it. “And that ain’t the point here Rarity.”
The unicorn frowned back at the earth pony. “No, the point of this is that somepony has to go up there and tell Rainbow Dash what’s going on before she decides to get all stabby! And as I am about to have a mental breakdown due to the premature death of one of my best friends, I nominate you!” Rarity said as she poked Applejack in the chest hard enough to make the farmer take a step back.
After staring at the unicorn for a few seconds, Applejack relented and made her way up the stairs to where Rarity’s bedroom was giving Rainbow Dash a perfect few of the nearly motionless undead horde. She took a few tentative looks around. “Uh...Rainbow, where are the girls?”
Dash didn’t look away from the window. “I had them go to Sweetie’s room to sew her sheets together. If it comes down to it, I can fly the three of them out of here and to Canterlot before I ask Celestia to like...scorch the earth around here or something.”
Applejack sighed in relief. While she didn’t think Rainbow would have hurt the girls under normal circumstances… No getting sidetracked, Applejack told herself. “Listen Rainbow,” the farm pony said before taking a step forward.
Only to be stopped when the pegasus held up a wing. “Applejack wait!”
“I...I don’t,” she said before a rather wet sniff came from Rainbow Dash. “I don’t want you to see me cry.”
Applejack blinked. “R-Rainbow?”
“It’s my fault...Applejack,” the rainbow mare said softly as she paused to shudder. “I...those cookies. The cookies that turned everypony into a zombie. They were...they were just supposed to be a joke. I didn’t know they would...I...I didn’t know…Oh Celestia! Everypony’s dead, and it’s is all my fault!”
The farm pony gulped as she got an uneasy feeling in her gut. This was not how this was supposed to be going. Rainbow Dash was supposed to be frightened, not sad! They were supposed to make her run all over town like a little scaredy pony and be sure to laugh about plum foolish she was for years afterwards.
A silence fell over the room as Applejack tried to think of how she was going to break the news to Rainbow. Then she wondered if Dash would be going to jail. This wasn’t like spilling water in a cloud factory. She had...Rainbow Dash had...killed another pony.
Because...
Rainbow cleared her throat. “Hey Applejack....when this is all over, can I borrow your rope?”
“Huh?” Applejack asked.
“It’s a good rope, you tied me up with it more than once,” she went on. “I think if I tie the one end around a strong tree, I won’t even feel it when my neck snaps.”
Applejack’s eyes widened in horror. “W-WHAT?”
“It’s how pegasi hang themselves. Since we got wings, we put the noose around our necks and-”
“IT’S ALL A PRANK RAINBOW!” Applejack shouted in a panic before Rainbow could take another step down that road.
The pegasus slowly turned around, looking at Applejack with wide eyes. “W-What? What...prank?”
Unable to look at her friend directly in the eyes, the farmer took off her hat and looked down at the ground. “It’s just a stupid prank Rainbow,” she said. “It wasn’t supposed to be like this! Y-You were supposed to run away n-not...not-”
“Smack Mrs Cake upside the head with a roller?” Rainbow asked.
Applejack nodded. “Y-Yeah.”
“Then have to stop from bashing her brains in when Pinkie Pie came out of the cabinet shouting for me to wait?” the pegasus asked evenly.
Applejack sighed and hung her head even lower. “Yeah.”
“Then have Pinkie explain this totally lame prank where you guys were going to make me think I poisoned and killed the entire town with a bunch of cookies that I had tested before hoof to make sure they were good enough to pass for real ones before getting some food coloring and making it look like I knifed the Cake family and Pinkie so you guys would think your prank worked too well and made me end up killing ponies so you were the ones with the mega guilt?” she asked quickly.
Applejack nod-“Wait!” she said as she looked up to see Rainbow Dash wearing the biggest shit-eating grin in all of history. “Oh....well...buck you too Rainbow.”
“I mean seriously girls? Mrs. Cake? Mrs. Cake?” Rainbow demanded as she paced through the air around the Table of Friendship in what Twilight Sparkle could only call a victory lap, one of several she had taken since their meeting the morning after the failed zombie prank. “We’ve fought evil alicorns, dragons, Discord, pony magic eating monsters, pretty much everything the Everfree Forest can throw at us, and you girls think I’m going to be afraid of MRS. CAKE?”
The Princess of Friendship groaned as she rubbed her temples. “Yes. I am well aware of the flaw in my plan involving your flight or fight response choosing the later rather than the former. Thank you for pointing it out to me FOR THE TENTH TIME!”
“And another thing,” Rainbow Dash went on. “Pranks aren’t supposed to be enjoyed by the ponies they get pulled on, but you get up, dust yourself off, learn to laugh at yourself and keep going because that’s all it’s supposed to be. I mean, remember that one prank you all pulled on me last time?”
Twilight raised an eyebrow. “Prank? What prank did we all pull on you last time?”
Rainbow raised an eyebrow. “You know, that one where you all dressed up as that purple pony in the cape and acted like a single hero so I would feel like a total loser when the town just completely forgot against me despite everything I’ve done for them over the years?” she said. “I mean, if that wasn’t a prank, I’d probably hate you all for the rest of my life and plot out this horrible revenge plan that brings about the end of Equestria or something. But since it was all in good fun, I just laughed it off. I mean seriously, teaching me a lesson while acting like a bunch of bucking hypocrites and creating a superhero identity whose sole purpose is to outshine me while talking about how heroes are supposed to only want to help ponies without getting any fame? How can you not laugh at a prank like that?”
Everypony’s eyes went wide.
“Wait, you mean the Mare Do Well thing? That wasn’t a-” was all Spike was able to say before Twilight gagged him with her magic.
“Oh look at the time Spike! Better start cleaning the west wing!” Twilight giggled as she levitated the little purple dragon through the nearest door at something approaching the speed of sound.
As Rainbow Dash looked back down at the table of ponies in confusion, Twilight lit up her horn to turn the map on. “So um...Friendship Mission anypony?” she asked with a nervous grin.
Author's Note
And to those of you going "But this isn't mature/dark/etc..."
Of course not. But a comedy tag would have made the ending too predictable. 
